The present invention relates to an anti-locking brake system for a motor vehicle.
Anti-locking brake systems prevent locking of the vehicle wheels under conditions of poor road adhesion and/or panic brake response by the driver, in order to maintain steerability of the vehicle and to achieve optimum stopping distance.
Different types of anti-locking brake systems are known, the so-called "individual wheel control" system is the most efficient, in that there is a control channel with a speed sensor and control valve assigned to each wheel. Thus, only the wheel that is detected as slipping is controlled by the anti-locking system, so that the non-slipping wheels are able to continue to provide braking effort to achieve the shortest possible stopping distance. Such a system is disclosed in German Patent DE-OS No. 30 26 254 (FIG. 1).
A disadvantage of "individual wheel control" anti-locking brake systems, however, is the occurrence of yaw or differential steering forces when braking is performed on a road with different coefficients of friction. These steering forces must be compensated by the driver through appropriate countersteering actions. Under poor conditions, such maneuvers can be extremely difficult.
For this reason, a "modified individual wheel control" anti-locking brake system has been developed, which is essentially similar to the "individual wheel control" system, but in which the differences in brake force between the two wheels of a common axle do not exceed a predetermined value. An example of such a "modified individual wheel control" system is disclosed in U.S. Pat. No. 4,313,166, incorporated herein by reference. With a "modified individual wheel control" system, the reapplication of braking pressure for the respective wheels of an axle occurs concurrently and the steering counter forces required to correct the yaw moments are considerably reduced.
Another type of known anti-locking brake control system is the "select-low control" system, in which the same brake pressure is released for both wheels of a given axle via a common control valve. Each wheel is arranged with a speed sensor. The corresponding electronic system controls the common regulation valve as a function of the dynamic condition of whichever wheel is running on the lower coefficient of friction road surface. The other wheel running on the higher coefficient of friction road surface is synchronously co-controlled with the wheel having the lower coefficient of friction, and is thus generally underbraked. While such an arrangement has the advantage of providing especially good lateral stability and steering control, particularly on road surfaces having laterally different coefficients of friction, the arrangement has the disadvantage or a longer braking distance.
The anti-locking brake system of the prior art, as shown in German Patent DE No. 30 26 254 (FIGS. 2 and 4), from which the present invention proceeds, exhibits an electronic control system that provides "individual wheel control," but where the regulating valves that control the braking pressure of the respective wheels of a given axle operate via a common multi-way valve that selects the regulating valve of the wheel running on the road surface having the lower adhesion level. In this manner, a "select-low control" characteristic is achieved for this axle using an electronic control system for "individual wheel control," which is desirable in certain cases. A disadvantage of this solution of the prior art lies in the expense incurred for the additional multi-way valve.